This invention relates to power generation; and more particularly to a power plant or air motor apparatus utilizing compressed air or liquid air for energy storage.
Fossil fueled power plants have been the mainstay of industry for over 100 years. While internal combustion engines have been firmly entrenched as the main power source for many applications, recent increases in the cost of crude oil have had a particularly devastating effect on industries that rely on engines powered by these fuels. Meanwhile, efforts to develop affordable alternatives to fossil fueled plants have proven challenging.
Electricity remains an attractive alternative power source. A reliable generation, transmission, and distribution system for electricity is already in place. Furthermore, electricity is generated from a diversity of sources including coal, nuclear, water (hydro), wind, and solar power. For most parts of the country, the regional cost of electrical energy is largely insensitive to the price of crude oil. Electricity has already been successfully used in certain transportation applications where the energy does not have to be stored—most notably, electric trains, buses, and trolleys. All of these vehicles obtain energy, as needed, from electrified rails or overhead cables. Unfortunately, electricity as a transportation power source has yet to gain widespread use in applications apart from electrified railways. The biggest challenge at this point is not to develop alternative methods of generating electricity, but to develop a means of storing the electrical power in a way suitable for use in the transportation sector. To be useful for common transportation applications, a technology would need to:                Be suitable for use on non-electrified roadways, waterways, and airways.        Provide sufficient range to the vehicle to make long journeys possible without refueling.        Offer an economically viable alternative to consumers in terms of the acquisition, operational, and disposal costs.        Offer reliable operation, and be easy to repair and maintain.        Be scalable.        Be safe to use.        Be environmentally friendly (by eliminating carbon emissions and minimizing any contribution to landfills.)        
While batteries are an obvious choice for electrical energy storage, they don't necessarily satisfy all of the requirements outlined above. There are alternatives to the use of chemical batteries for the bulk storage of electrical energy. One such alternative is to store air (in a compressed gas or liquid form) and use it as needed. Much like batteries which convert electrical energy to chemical potential energy (and back again), so also will a compressed-air energy storage (CAES) system convert electrical energy to compressed-air potential energy (and back again). While a battery generates electricity on demand by a chemical reaction, so also will a CAES system generate electricity on demand by releasing compressed air to drive an electrical generator. Unlike the battery, the CAES system also has the option of using the pressurized air directly to drive an air motor and perform mechanical work (with or without accompanying electrical generation).
The use of compressed air as an alternative means of storing energy is not new. U.S. Pat. No. 8,481 (dating from the 1850's) describes an early air engine. Many developments have occurred since that time; but with the historical affordability of crude oil, and the dependability and effectiveness of the internal combustion engine in performing work, there has been little incentive for industry to develop other technologies, such as CAES, as an alternative power source. While the internal combustion engine became the dominant technology for transportation, air motors have found use in other applications such as for portable tools. In their development, air motors gained a reputation for affordability and longevity; but, unfortunately, also for inefficiency.
For compressed air to be considered a viable means of energy storage for transportation purposes, the air motor must be efficient enough to reach transportation design goals in a manner similar to that of the internal combustion engine. Without sufficiently high efficiency, the volume of air that must be transported would be prohibitive for most applications, and CAES will remain an unattractive design alternative.
Most engines (internal combustion engines, jet engines, and the like) burn a fuel which transfers heat into the gas. The resulting rise in gas temperature causes an increase in gas pressure. The design of the engine allows this increase in pressure to be traded for an increase in volume and the force of the gas acting on a movable surface translates to work done by the engine.
Air motors, in contrast to internal combustion engines, don't burn fuel; but instead route air from a high-pressure storage tank into a cylinder to pressurize the cylinder. At high rpm, the compressed gas expands rapidly (following essentially an adiabatic process) and cools dramatically as it expands. The exhaust expelled from the motor can be cryogenic. Such temperatures impose a limit on the efficient operation of the air motor because, at cryogenic temperatures, air loses pressure and consequently its effectiveness as a propellant.
U.S. Pat. No. 1,926,463 describes a compressed air motor which warms cold exhaust air using a heat exchanger. This prepares the air for use by a subsequent stage, but does nothing for the air in the original cylinder as it undergoes a power stroke. In a similar manner, U.S. Pat. No. 7,296,405 transfers air from one cylinder to another in an effort to counteract the cooling effect of expansion. A drawback in this process is that, in doing so, the system has to expend energy to compress the ambient air. A similar drawback is found in the systems described in Japanese publications JP02245401 A2 and JP07314315 A2, as well as publications WO9504208 A1 and EP0666961 A1.
U.S. Pat. No. 4,311,917 describes a system that consumes liquid air and uses it to generate electricity, charge batteries, and power an assembly. While the invention described herein also connects systems together to create a power plant, it is of a significantly different design than that described in the U.S. Pat. No. 4,311,917.
U.S. Pat. No. 4,432,203 describes use of an atomized spray of water to affect the temperature of the working fluid. As described therein, water is sprayed into a hot chamber where it immediately flashes into steam. The resulting hot water vapor transfers heat to air in an expansion chamber. In contrast, the present invention uses water near its ambient temperature to warm air in an expansion chamber which would otherwise be near cryogenic. No fuel is burned in the air motor of the present invention to create heat as is necessary in the U.S. Pat. No. 4,432,203.
Modern, large-scale CAES systems are used for the bulk storage of power by electric utilities. As such, the CAES system serves as a peaking power plant to stabilize the utility's electric grid. The systems are large systems typically requiring an underground cavern in which to store the compressed air. Most such systems also use a process which burns fuel to heat the air as the pressurized air is converted to mechanical energy. The present invention could be used as a CAES system; however, it has the advantage of avoiding the burning of fuel. Also, a major drawback of current CAES technology is that large tanks (or sealed underground caverns) are required to store the pressurized air. A liquid air energy storage (LAES) system such as taught by the present invention avoids this difficulty by storing the bulk of the air as a liquid rather than as a gas. Efficient production of liquid air requires cool, compressed air, and the present invention can be readily used for the production of liquid air. Liquid air is easily converted to a gaseous compressed air by warming, and air engines have been designed to use gaseous air, liquid air, or both.